This invention is concerned with a method for recording high quality images on electrophotographic film.
There presently exist a large variety of electrostatic image recorders which rely for their operation on certain well-known basic steps. First an electrical charge is applied to a previously discharged photoconductive medium after which the medium is exposed to a light pattern to form a latent image thereon in the form of incremental areas which remain charged and other incremental areas which are discharged. The initial charge places a great number of electrons at or slightly below the surface of the medium. The light pattern furnishes photons which cause the electrons to migrate toward an ohmic member which normally comprises a layer of some conductor below the photoconductive medium. Increments of the medium which are subjected to high intensities of light will discharge their electrons more rapidly and fuller than those increments which are subject to lesser light intensities. The totally unilluminated increments theoretically will not discharge their electrons at all, although, as will be seen, there is a continuous discharge which occurs even in total darkness, the degree of which is dependent upon the nature of the photoconductive medium.
Continuing with the basic steps which are known, after exposure, there exists on the photoconductive medium a latent image which is a duplication of the pattern to which the medium has been exposed. The dark increments have the greater retained charge and the light increments have the lesser retained charge. Toner is then applied to this latent image to make it visible, the toner comprising fine particles of carbon, resin and the like which are electrophoretic in nature and hence attracted to the charged increments and not to the uncharged increments. The range of grey tones capable of being achieved depends upon the ability of the medium to retain a gradient of charge between the extremes of dead black and dead white (full charge and total absence, respectively). So far as is known, available photoconductive mediums are incapable of achieving the grey scale which can be achieved by the electrophotographic film which will be described herein.
Once the toner has been applied to the latent image the image becomes visible and is either capable of being transferred to another member (xerography) or fused in place on the photoconductive medium (electrofacsimile, called electrofax). In the first instance, image copiers utilize a member which comprises a metal drum having a surface of amorphous selenium, the toned image being transferred to a sheet of paper and fused thereon. In the second instance, image copiers utilize electrophotographic members which comprise sheets of conductive paper having coatings of zinc oxide-resin mixtures. The latent image is formed directly upon the photoconductive surface of the paper, passed through a toner bath and thereafter fused in place.
The same techniques have been proposed for the production of images of photographic quality, such as for example, the production of microimages. The problems with these known techniques that prevent this include the inability to produce high resolution images. For example, where an image is to be magnified on the order of twenty times for reproduction or viewing or printing, imperfections likewise are magnified. To obtain a resolution in the enlarged image of five lines per millimeter the basic image must have a resolution of one hundred lines per millimeter. Systems proposed heretofore have not been capable of achieving such resolution, so far as is known. In the system proposed herein, resolutions of the order of one thousand lines per millimeter are practically achievable.
Another aspect of the problems lies in the quality of the images which can be achieved by xerographic and electrofax techniques. In high quality photography the film speed enables the capture of moving scenes and a great range of tones enables the reproduction of images with natural appearance. The known xerography and electrofax techniques mentioned are slow, have limited tone gradients and produce extremely contrasty images. They cannot be favorably compared even with low quality photographic film. Their use in microimaging is thus obviated because they cannot produce the basic requirements of microimages.
According to the invention, all of these difficulties are overcome by the use of the electrophotographic film of the type identified herein and by the techniques which will be detailed hereinafter.
Heretofore, the various steps in electrophotography have been treated as a succession of static, unrelated events. First, the photosensitive medium is charged, then it is exposed to an image. Light falls on the portions of the medium corresponding to the light areas of the image, causing the charge on those areas to dissipate while those portions of the medium corresponding to the dark areas of the image retain their charge. In this way, a latent photographic image is formed on the medium. Following this, toner is applied to the medium which tends to adhere to those portions thereof which still retain an electric charge, thereby reducing the latent image to visible form. Finally, the toner is fused to the medium so that the image thereon becomes permanent or the image is transferred to another member where it is fused.
The aforesaid steps are performed in successive time intervals, usually at different locations in the reproduction apparatus. There is no relation between the times of the various steps.
More significantly, since the prior processes are concerned with impressing a relatively low resolution image on a relatively large image area, they operate at relatively slow copying speeds, i.e. 2 to 10 seconds.
A typical photoconductive medium such as selenium has a characteristic dark decay curve. Once charged to its customary initial voltage, e.g. 500-600 volts, it exhibits a fairly rapid rate of decay, e.g. 50-100 volts/min. during the first minute or so. Then the rate of decay gradually becomes less until the surface potential reaches a substantially constant residual background value of about 30-50 volts. Conventional xerography systems, being fairly slow as noted above, process the medium at a time when the rate of decay of the charge on the medium is fairly slow.
The development of high quality images, especially on microfilm, requires an entirely different approach. The area to be imaged may be very small although as seen the invention herein is not so limited. Also, the resolution and tonal range requirements of imaging intended to compete with high speed, high resolution photography are much higher than is the case with larger xerographic or electrofax prints.
Thus, in contrast to the foregoing, the present technique involves controlling the steps of a reproducing process as a dynamic series of interrelated events, some of which are performed concurrently and all of which are performed on a greatly collapsed time scale as compared with conventional xerographic or electrofax processes. The voltages which are involved are substantially less than those used in prior processes although due to the techniques used, the field strength per unit thickness is much higher than that of prior art members. In the film used it is approximately 10.sup.6 volts per centimeter although the maximum surface potential to which the electrophotographic film will be charged is of the order of 50 volts. Note that in prior art photoconductive members the order of surface potential is 500 to 600 volts and the noise voltage is of the order of 50 volts.
The important steps according to the invention are all carried out at a period of time when the charge on the medium is decaying most rapidly, that is, very early on the characteristic dark decay curve of the electrophotographic medium. For best results it is essential that the photoconductive film being processed have high speed, that is a high electronic gain and a high ratio between dark and light resistivity. Applicant has developed an electrophotographic film for this purpose. A short description of this electrophotographic film will suffice to provide an understanding of its relationship to the method of the invention.
The electrophotographic film developed by applicant is based upon a polyester substrate of the type sold by E. I. DuPont de Nemours Company as "Mylar" having a thickness of about 0.005" and being transparent and quite flexible. After normal outgassing and radioactive brushing, an ohmic coating such as indium oxide is sputtered by R.F. plasma sputtering techniques on the substrate to a thickness of about 500 Angstroms. Then a coating of photoconductive material is sputtered onto the ohmic coating by means of R.F. plasma sputtering, but using a special bias circuit in the power supply. The material which has been successfully coated is cadmium sulfide at a thickness of the order of 3000 Angstroms.
The coating which results is flexible, transparent, n-type, hard as glass and abrasion resistant, has extremely high gain and hence high speed, is oriented crystalline and has a dark resistivity of about 10.sup.12 ohm centimeters. Its light resistivity is about 10.sup.8 ohm centimeters, thus giving a ratio of about 10.sup.4.
Other materials named in the said copending application, Ser. No. 378,180, may be used to produce an electrophotographic film for use in the method of this invention, but the most satisfactory thus far has been the one utilizing cadmium sulfide.
No other electrophotographic film is known at this time which is the equivalent of the above-mentioned, but others may exist which have sufficient of the characteristics to utilize the method of the invention.
One of the most important aspects of the invention is the utilization of the variable sensitivity of the electrophotographic film to achieve excellent results and quality in the eventually reproduced images almost without regard to the light conditions. Specifically, the method of the invention contemplates that the voltage to which the electrophotographic film is charged will be determined by the incident light, that is, the average lighting conditions of the image to be reproduced. So far as known, this has not been utilized in any prior art method. Most, if not all, reproducing machines for copying (so far as known, no xerographic or electrofax apparatus is used commercially for photography) utilize brilliant lights of a fixed intensity to illuminate the material being copied. The prior art photoconductive media are insufficiently sensitive to suggest that the sensitivity of the photoconductive medium may be varied in accordance with the ambient light. There is no variation in the charging voltages.